The present invention relates to guest-host liquid crystal display devices, and in more particular, to guest-hose liquid crystal display devices utilizing a cholesteric-nematic phase transition.
It has been already known that the application of an electric field to a mixture of a host nematic liquid crystal and a guest pleochroic dye causes the guest dye molecules to align with the applied electric field and can control the absorption of incident light by the guest molecules. The guest-host liquid crystal display device utilizing the above fact has been proposed by G. H. Heilmeiyer and L. A. Zanoni, Applied Physics Letters, 13, 91, 1968. The proposed device includes a polarizer, a liquid crystal having the dipole moment in the direction of the longitudinal molecular axis, that is, having the so-called positive dielectric anisotropy, a pleochroic dye mixed in the liquid crystal, and a pair of transparent electrodes for applying an electric field to the liquid crystal. In the absence of the applied electric field, the dye molecules are irregularly oriented, and absorb a specified wavelength component of the incident light transmitted through the polarizer to produce the colored light (colored state). When the electric field is applied to the liquid crystal, the dye molecules align in parallel to the applied electric field together with the liquid crystal molecules, thereby producing substantially the non-colored light (non-colored state). However, the above device is unsatisfactory in brightness for display and inferior in viewing angle dependence because of the use of the polarizer.
In addition to the conventional guest-host type liquid crystal display devices of this kind, other guest-host display devices have been proposed in which a helical molecular ordering structure (a cholesteric condition) is established in a nematic liquid crystal by adding thereto an optically active material. Such guest-host liquid crystal display devices are disclosed in U.S. Pat. Nos. 3,833,287 and 3,837,730 which do not employ any polarizer. Further, in the above devices, the molecular axes of a guest pleochroic dye dissolved in the host material assume a helical molecular ordering in accordance with the helical molecular ordering structure of the host material. The helical axis is directed to the electrode, that is, lies in the direction parallel to an applied electric field.
A cholesteric liquid crystal phase, in which the helical axis of the helical molecular ordering structure is parallel to the applied electric field, is called a gradjean condition. In the absence of an applied electric field, the guest and host materials are put into a grandjean condition, and thus the guest material absorbs incident light to put the guest and host materials in a relatively colored state. On the other hand, when an electric field is applied, the helical molecular ordering or twisting is reversed to reorient the guest and host molecules in parallel to the applied electric field. Thus, the incident light is scarcely absorbed by the guest material, thereby putting the guest and host materials in a substantially non-colored state.
Besides the grandjean condition, there is another cholesteric liquid crystal phase in which the helical axis of the helical molecular ordering structure is perpendicular to the applied electric field. Such a phase is generally called a focalconic condition, and a display cell such as having a cholesteric phase which are put in a focalconic condition in the absence of an applied electric field, is known. An article entitled "New absorptive mode reflective liquid-crystal display device" by Donald L. White and Gary N. Taylor (Journal of Applied Physics, Vol. 45, No. 11, Nov., 1974, pp 4718-4723) disclosed and teaches the following. The guest and host materials which exhibit a grandjean condition and a focalconic condition in the initial state having no applied electric field, undergo a phase transition by the application of an electric field into a so-called nematic phase or a homeotropic condition in which the guest and host molecules are aligned parallel to the applied electric field. The transition voltage V.sub.T necessary for the phase transition from the grandjean or focalconic condition to the homeotropic condition is given by the following equation V.sub.t =1.39D/P, where P indicates the pitch of helical molecular ordering structure when the guest and host materials exhibit a cholesteric phase, and D the gap of display cell. In detail, White and Taylor shows P.ltoreq.6.7 .mu.m and D=12 .mu.m, i.e., D/P.gtoreq.4. Furthermore, an article by H. S. Cole, Tr and S. Aftergut, "Applied Physics Letters" Vol. 31, No. 2, July 15, 1977 shows about D/P.gtoreq.2 and that contrast ratio increases as a ratio of D/P increases.